Superior and inferior vena caval flows during respiration: pathogenesis of Kussmaul's sign

Respiratory induced changes in superior (QSVC) and inferior (QIVC) vena caval flows and abdominal pressures were evaluated in anesthetized closed-chest dogs. QSVC and QIVC were measured with ultrasound transit time flow probes (n = 5), and general (Pab) and regional subdiaphragmatic (Pd) abdominal pressures were measured by air-filled balloons (n = 5), during two respiratory maneuvers produced by phrenic nerve stimulation, i.e., simulated spontaneous inspiration (SSI), and Mueller maneuver (MM), with the airway occluded to minimize diaphragmatic descent. With hypervolemia: during SSI, QSVC decreased, QIVC increased, and right atrial pressure increased (P less than 0.05) despite a decrease in esophageal pressure (Pes), i.e., Kussmaul's sign; during MM, both QSVC and QIVC increased (P less than 0.05) without Kussmaul's sign. The ratios delta Pab/delta Pes and delta Pd/Pes were larger during SSI than MM (P less than 0.01). With hypovolemia: during SSI and MM, QSVC increased and QIVC decreased with a venous pressure gradient across the diaphragm (P less than 0.05), consistent with development of a vascular waterfall. These results suggest that 1) changes in abdominal pressure may affect the patterns of QSVC and QIVC during respiration, depending on blood volume status; 2) a prolonged inspiration with hypovolemia may decrease QIVC because of the development of a vascular waterfall; 3) QSVC and QIVC may be interdependent during respiration; and 4) the essential mechanism of Kussmaul's sign is a substantially larger inspiratory increase in abdominal pressures produced by diaphragmatic descent compared with the decrease in intrathoracic pressure under hypervolemic conditions, rather than the presence of pericardial pathology or right heart dysfunction.